The human spine serves many functions. The vertebral members of the spinal column protect the spinal cord. The spinal column also supports other portions of the human body. Furthermore, moveable facet joints and resilient discs disposed between the vertebral members permit motion between individual vertebral members. Each vertebrae includes an anterior body and a posterior arch. The posterior arch includes two pedicles and two laminae that join together to form the spinous process. A transverse process is laterally positioned at the transition from the pedicles to the laminae. Both the spinous process and transverse process provide for attachment of fibrous tissue, including muscle. Two inferior articular processes extend downward from the junction of the laminae and the transverse process. Further, two superior articular processes extend upward from the junction. The articular processes of adjacent vertebrae form the facet joints. The inferior articular process of one vertebra articulates with the superior articular process of the vertebra below. The facet joints are gliding joints because the articular surfaces glide over each other.
Vertebral implants are often used in the surgical treatment of spinal disorders such as degenerative disc disease, disc herniations, curvature abnormalities, and trauma. Many different types of treatments are used. In some cases, spinal fusion is indicated to inhibit relative motion between vertebral bodies. Spinal fusion often involves the removal of the vertebral disc and insertion of an interbody implant to create a fused junction between a pair of vertebral bodies. Furthermore, the facet joints may be fused to complete the fusion between vertebral pairs. Facet fusion often involves destruction of the facet by decorticating the opposing articulating surfaces and packing bone growth promoting substances such as grafts or synthetic materials into the space between the articular processes. The facet joints are generally small as compared to the intervertebral space. Consequently, limited amounts of bone-growth promoting substances may be inserted into the joint. Some of the bone-growth promoting substances tend to disperse post-operatively resulting in a less robust fusion. Furthermore, the overlying fibrous tissue may further disperse the bone-growth promoting substances as a result of contact, friction, and/or the ingrowth of fibrous mass. These and other factors may result in pseudarthrosis or inadequate fusion.
In the spinal surgery field, surgical procedures are often performed to correct problems with displaced, damaged or degenerated intervertebral discs due to trauma, disease or aging. Bone graft materials are often used in spine fusion surgery. Current spinal fusion implants utilize grafts of either bone or artificial implants to fill the intervertebral disc space.
While generally effective, the use of bone grafts has some limitations. Autologous bone grafts, being obtained from the patient, require additional surgery and present increased risks associated with its harvesting, such as risk of infection, blood loss and compromised structural integrity at the donor site. Bone grafts using cortical bone remodel slowly because of their limited porosity. Traditional bone substitute materials and bone chips are more quickly remodeled but cannot immediately provide mechanical support. In addition, while bone substitute materials and bone chips can be used to fill oddly shaped bone defects, such materials are not as well suited for wrapping or resurfacing bone. Indeed, the use of bone grafts is generally limited by the available shapes and sizes of grafts provided.
With regards to bone grafts, allograft bone is a reasonable bone graft substitute for autologous bone. It is readily available from cadavers and avoids the surgical complications and patient morbidity associated with harvesting autologous bone. Allograft bone is essentially a load-bearing matrix comprising cross-linked collagen, hydroxyapatite, and osteoinductive bone morphogenetic proteins. Human allograft tissue is widely used in orthopaedic surgery.
Many approaches using allograft implants in a facet fusion involve mineralized pieces of allograft that are threaded across the joint or impacted into place. These cortical allograft implants can take a very long time to attach and incorporate with the host bone ultimately resulting in a fusion. These allograft implants also require specialized preparation of the facet joint, such as decortication, for the cortical bone implants to fit into place. Many times the preparation of the joint results in the removal of a significant amount of the facet joint leading to further destabilization.
Accordingly, it would be desirable to construct an implant, particularly an interbody implant, to better utilize the benefits of allograft treatment.